Bottom Line:
It remarkably deepens isotherm at deep layers by the amplitude of ~120 m and induces a maximal velocity amplitude about 0.18 m/s, which is far larger than the median velocity (0.02 m/s).The deep eddy is generated in a wake when a steering flow in the upper layer passes a seamount, induced by a surface cyclonic eddy.Deep eddies significantly increase the velocity intensity and enhance the mixing in the deep ocean, also have potential implication for deep-sea sediments transport.

ABSTRACTDespite numerous surface eddies are observed in the ocean, deep eddies (a type of eddies which have no footprints at the sea surface) are much less reported in the literature due to the scarcity of their observation. In this letter, from recently collected current and temperature data by mooring arrays, a deep energetic and baroclinic eddy is detected in the northwestern South China Sea (SCS) with its intensity, size, polarity and structure being characterized. It remarkably deepens isotherm at deep layers by the amplitude of ~120 m and induces a maximal velocity amplitude about 0.18 m/s, which is far larger than the median velocity (0.02 m/s). The deep eddy is generated in a wake when a steering flow in the upper layer passes a seamount, induced by a surface cyclonic eddy. More observations suggest that the deep eddy should not be an episode in the area. Deep eddies significantly increase the velocity intensity and enhance the mixing in the deep ocean, also have potential implication for deep-sea sediments transport.

f1: The bathymetry of the northern SCS.(a) Colors show the bathymetry of the northern SCS. Triangle and square are locations of Moorings A and B, respectively. The vectors are geostrophic currents on 18 April 2012 from altimeter data. The altimetry data over the shelf shallower than 100 m are masked. (b) The topographic map from the bathymetry (color) in Xisha area marked by box in (a). (c) The water depth profile along the section marked by red line in (b). Maps are generated using Global Mapper v12.

Mentions:
Surface eddies are frequently observed in the Ocean and have substantial impact on physical and biogeochemical budget123456. However, due to lack of observation, deep eddies are still poorly understood and less studies focused on the phenomena789101112. The South China Sea (SCS) is known for the presence of strong eddy activity13. The northern SCS has a broad continental shelf and a steep continental slope (Fig. 1a) with numerous seamounts. A strong boundary current exists in the northern SCS and flows southward near the Xisha area14. Surface mesoscale eddies are frequently observed near the Xisha Islands, including eddies locally generated and propagated from the northeastern and eastern regions as well15. A strong interaction among currents, eddies and topography is expected in the area.

f1: The bathymetry of the northern SCS.(a) Colors show the bathymetry of the northern SCS. Triangle and square are locations of Moorings A and B, respectively. The vectors are geostrophic currents on 18 April 2012 from altimeter data. The altimetry data over the shelf shallower than 100 m are masked. (b) The topographic map from the bathymetry (color) in Xisha area marked by box in (a). (c) The water depth profile along the section marked by red line in (b). Maps are generated using Global Mapper v12.

Mentions:
Surface eddies are frequently observed in the Ocean and have substantial impact on physical and biogeochemical budget123456. However, due to lack of observation, deep eddies are still poorly understood and less studies focused on the phenomena789101112. The South China Sea (SCS) is known for the presence of strong eddy activity13. The northern SCS has a broad continental shelf and a steep continental slope (Fig. 1a) with numerous seamounts. A strong boundary current exists in the northern SCS and flows southward near the Xisha area14. Surface mesoscale eddies are frequently observed near the Xisha Islands, including eddies locally generated and propagated from the northeastern and eastern regions as well15. A strong interaction among currents, eddies and topography is expected in the area.

Bottom Line:
It remarkably deepens isotherm at deep layers by the amplitude of ~120 m and induces a maximal velocity amplitude about 0.18 m/s, which is far larger than the median velocity (0.02 m/s).The deep eddy is generated in a wake when a steering flow in the upper layer passes a seamount, induced by a surface cyclonic eddy.Deep eddies significantly increase the velocity intensity and enhance the mixing in the deep ocean, also have potential implication for deep-sea sediments transport.

ABSTRACTDespite numerous surface eddies are observed in the ocean, deep eddies (a type of eddies which have no footprints at the sea surface) are much less reported in the literature due to the scarcity of their observation. In this letter, from recently collected current and temperature data by mooring arrays, a deep energetic and baroclinic eddy is detected in the northwestern South China Sea (SCS) with its intensity, size, polarity and structure being characterized. It remarkably deepens isotherm at deep layers by the amplitude of ~120 m and induces a maximal velocity amplitude about 0.18 m/s, which is far larger than the median velocity (0.02 m/s). The deep eddy is generated in a wake when a steering flow in the upper layer passes a seamount, induced by a surface cyclonic eddy. More observations suggest that the deep eddy should not be an episode in the area. Deep eddies significantly increase the velocity intensity and enhance the mixing in the deep ocean, also have potential implication for deep-sea sediments transport.